1. Field of the Invention
The present invention relates to a projector which enlarges an original image formed using a liquid crystal panel or the like and projects the enlarged image onto a projection surface, such as a screen, and in particular to a projector forming the original image with a plurality of reflective liquid crystal panels.
2. Description of the Related Art
An image projection apparatus combining reflective liquid crystal display elements and polarization beam splitters is disclosed in Japanese Patent Application Laid-Open No. 2001-154268. In Japanese Patent Application Laid-Open No. 2001-154268, an image projection apparatus made of a white light source 1001, reflective liquid crystal display elements 1002R (reflective liquid crystal display element forming a red original image), 1002G (reflective liquid crystal display element forming a green original image), and 1002B (reflective liquid crystal display element forming a blue original image) forming red (R), green (G) and blue (B) original images, and a projection optical system 1003, as shown in FIG. 12, includes a dichroic mirror 1004, a color separation system and a color combination system. The dichroic mirror 1004 is arranged between the white light source 1001 and the reflective liquid crystal display elements 1002R, 1002G and 1002B. The color separation system includes first and second polarization beam splitters 1005 and 1006 arranged between the dichroic mirror 1004 and the reflective liquid crystal display elements 1002R, 1002G and 1002B. The color combination system includes the first and second polarization beam splitters 1005 and 1006 as well as a third polarization beam splitter 1007, which are arranged between the reflective liquid crystal display elements and the projection optical system.
Here, color separation with the polarization beam splitters is performed through the association of color components (R, B) with polarization directions (P, S) by providing between the dichroic mirror 1004 and the second polarization beam splitter 1006 a first color selective wave plate 1008 which can rotate the polarization direction of light of a predetermined wavelength region (here, light in the wavelength region of blue) by 90° and by providing between the second polarization beam splitter 1006 and the third polarization beam splitter 1007 a second color selective wave plate 1009 (here, one which rotates the polarization direction of light in the wavelength region of blue by 90°). Thus, the polarization direction of blue light is rotated 90° by the first color selective wave plate 1008, the blue light is turned into P-polarized light, the red light is turned into S-polarized light, the red and blue light are incident on the second polarization beam splitter 1006, and are separated into an optical path (R) of second color light and an optical path of third color light.
Moreover, in a first optical path, the light reflected by the first polarization beam splitter 1005 has its polarization direction rotated 90° and is reflected by the first reflective liquid crystal display element 1002G, is transmitted by the first polarization beam splitter 1005, has its polarization direction rotated 90° by a ½ wave plate 1012, is reflected by the third polarization beam splitter 1007, and reaches the projection optical system 1003. The light of the second optical path has its polarization direction rotated 90° and is reflected by the second reflective liquid crystal display element 1002R, and is transmitted by the second polarization beam splitter 1006. The light of the third optical path has its polarization direction rotated 90° and is reflected by the third reflective liquid crystal display element 1002B, and is reflected by the second polarization beam splitter 1006. At the second polarization beam splitter 1006, the optical paths of the two colors red and blue (R, B) are combined into one, the polarization direction of the blue light is rotated 90° by the second color selective wave plate 1009, both the red and the blue light become P-polarized light, are transmitted by the third polarization beam splitter 1007, and reach the polarization optical system 1003, thus combining the light of the three colors.
In this conventional example, polarizing plates 1010 and 1011 are respectively arranged on the incidence side of the first polarization beam splitter 1005 and the incidence side of the first color selective wave plate 1008, and unnecessary polarized components included in the illumination light from the light source 1000 are cut, thus increasing the contrast.
Regarding the performance of the polarizing plates, Japanese Patent Application Laid-Open No. 2001-154268 mentions, that a maximal transmittance of 1% for light of unnecessary polarization directions is necessary. In order to attain such high characteristics, dye-based polarizing plates with high heat resistance must be used for the polarizing plates used in image projection apparatuses using a liquid crystal. However, even though dye-based polarizing plates have a sufficient polarization action of absorbing unnecessary polarized components in the wavelength region of white light, they have the problem that their transmittance of transmitted polarized components is low, and their brightness is lowered significantly when the contrast is increased. As a countermeasure, the wavelength range over which the conventional example has the polarization action of absorbing unnecessary polarized components is limited to a predetermined range, thus trying to prevent a decrease of the transmittance.
However, what causes a decrease in contrast in the color separation and combination systems of the image projection apparatus of this conventional example is not only the degree of polarization of the light from the light source discussed in this conventional example, but also the angular characteristics of the polarization beam splitting film of the polarization beam splitters. Even in polarization beam splitting films which are designed such that they have a polarization separation capability of substantially 100% with respect to a predetermined incidence angle (45°), for incidence angles which deviate from the predetermined angle, the polarization separation performance deteriorates. In particular in order to attain a bright image, the range of incidence angles of the illumination light flux on the polarization beam splitting film must be increased, so that the influence of the angular characteristics is considerable.
Thus, in order to realize high contrast in image projection apparatuses using reflective liquid crystal display elements, it is necessary to eliminate unnecessary polarization components over the entire wavelength region, and there was the problem that a high contrast cannot be realized when polarizing plates as in the conventional example are used in which the characteristics of wavelength plates which are used for the optical paths of red (R) and blue (B) are limited for the blue wavelength region, and any polarization direction is transmitted in the red region.